Electrical resistivity tomography for the evaluation of areal block proportion (ABP) in bimunits: Modelling and preliminary field validation

The measurement and estimation of the Areal Block Proportion (APB) and of the Linear Block Proportion (LBP) are the basis for the estimation of the Volumetric Block Proportion (VBP), which is a fundamental parameter for the mechanical and technical characterization of bimunits. Existing approaches for ABP estimation are strongly conditioned by availability of accessible outcrops and/or borehole data. To address this problem, we propose a methodology based on non-invasive geoelectrical surveys. We realized numerous numerical simulations of the resistivity field on simplified geological sections across bimunits, characterized by different internal organizations of hard (high resistivity) blocks in a softer clay-rich (low resistivity) matrix, aiming at investigating the relationship between resistivity and ABP. We compared the results of the numerical simulations with real electrical resistivity tomographies (ERTs) acquired in two representative test sites in bimunits, providing a preliminary validation of modelled data. Our findings allowed us to define an equation relating the ABP with the electrical resistivity, which offers the possibility of estimating the ABP through non-destructive geo-electrical surveys, favoring their correlation with outcrop and/or borehole data and supporting their interpretation.